Thermionic generator having auxiliary anodes in the main discharge space



spt. 17, 1968 v GABOR ET AL 3,402,313

THERMIONIC GENERATO R HAVING AUXILIARY ANODES IN THE IN DISCHARGE SPACEed May 10, 1965 6 PULSE 4 a 5 3 4 6ENRATOR mgiwiw 2 United States Patent3,402,313 THERMIONIC GENERATOR HAVING AUXILIARY ANODES IN THE MAINDISCHARGE SPACE Dennis Gabor, London, England, and John Anthony Nil son,Montreal, Quebec, Canada, assignors to National Research DevelopmentCorporation, London, England, a British corporation Filed May 10, 1965,Ser. No. 454,319 Claims priority, application Great Britain, May 12,1964, 19,806/ 64 4 Claims. (Cl. 313-306) ABSTRACT OF THE DISCLOSURE Athermionic generator having, in addition to a heated emitter and acollector, a plurality of auxiliary anodes in the emitter-collectorspace. The auxiliary anodes, which are carried by and insulated from thecollector, provide a discharge to break down the electron space chargein the main discharge. Several methods of making combined collector andauxiliary anode structures are described. The auxiliary anodes are ofvery small area, and may terminate in sharp points. The discharge spacemay contain inert gases. The auxiliary anodes are preferably excited bypulses whose duration is considerably longer than the intervals betweenthe pulses. The main discharge current can be controlled by varying theauxiliary anode current.

This invention relates to thermionic generators and is more particularlyconcerned with controllable thermionic generators.

A thermionic generator is a device for the direct consion of heat intoelectrical power. It comprises an electron emitter which is heated toprovide free electrons which flow to a colder electron collector spacedapart from the emitter and thereby maintain a potential diiferencebetween the emitter and collector. In a controllable thermionicgenerator an auxiliary discharge is provided which produces ions tobreak down the electron space charge in the main discharge.

In one kind of controllable thermionic generator an auxiliary anode isprovided in a special discharge space which communicates with the maindischarge space through foraminations in the collector electrode.Alternatively an auxiliary cathode may be provided to produce theauxiliary discharge.

In accordance with the invention a thermionic generator has a pluralityof auxiliary anodes provided in the main discharge space, whichauxiliary anodes provide an ionizing discharge.

In order that the auxiliary discharge shall enhance the main dischargeit is desirable that the total area of the auxiliary anodes is as smalla proportion as possible of the emitter area which they serve. The totalarea of the auxiliary anodes should not exceed A of the emitter area andpreferably it should be less than this.

In a preferred arrangement the auxiliary anodes are energised withpositive going pulses in which the ontime is sufiicient to produce anionizing discharge and the off-time is of considerably longer durationbut is insufiicient to allow an appreciable decrease in the maindischarge current during the off-time. It has been found experimentallythat in generators operating in atmospheres of argon, krypton or xenonthe pulse repetition rate should be 5-10 kc. per cycle or higher and theduration of each pulse should be of the order of 1-5 microseconds.

In order to assist in the stabilisation of the discharge among theplurality of auxiliary anodes individual anodes resistors may beincluded in the current path to each auxiliary anode.

The invention will be better undesrtood with reference to the drawingsaccompanying this specification in which:

FIG. 1 is a schematic cross section of a thermionic generator embodyingthe invention, showing three different types of auxiliary anodes,

FIG. 2 shows one method of connection of the auxiliary anodes to acommon terminal,

FIG. 3 is a diagram illustrating pulse operation,

FIG. 4 is a diagram of the electrical characteristics of a thermionicgenerator,

FIGS. 5a to Si and 6a to 60 illustrate the stages in the manufacture oftwo different types of auxiliary anode systems,

FIG. 7 and FIG. 8 show alternative arrangements of electron collectorsembodying the invention.

Referring now to FIG. 1 there is shown therein the essential elements ofa thermionic generator comprising a heated emitter electrode 1 andspaced apart therefrom a collector electrode 2. The two electrodes arecontained in a vacuum tight envelope containing inert gases such asargon, krypton and xenon at 2 pressures of the order of 0.1-5 torr ormixtures of these gases with metal vapours. The collector electrode 2may preferably comprise part of such a vacuum envelope and may beimmersed in a coolant. The collector electrode 2 carries auxiliaryanodes 3 which are insulated from electrode 2 by means of beads 4 of asuitable insulator such as an enamel. Wires 3 may be centered in thebores by means of beads or tubes of ceramic material, not shown. Wires 3preferably consist of refractory materials such as tungsten, tantalum ormolybdenum.

FIG. 1 shows three alternative arrangements of auxiliary anodes 3. Inthe left-hand position electrode 3 and bead 4 are ground flush with thesurface of collector electrode 2 so as to give the advantage of a verysmall and well defined area of auxiliary anode. In the centre positionthe wire 3 and bead 4 project a little into the discharge space. Forinstance if the gap between the emitter 1 and the collector 2 is 2 mm.,which has been found to be an advantageous dimension in experiments, thewire may project to within 0.5-1.0 mm. from the emitter. This had theadvantage that the anode drop region in which the ions are produced isabout midway between the emitter and the collector, which favours theemitter in the distribution of ions, where ions are most needed. In thethird type, the wire ends in a sharp point. This can be produced, e.g.by electrolytic etching of tungsten of molybdenum wires. It has theadvantage of a more gradual development of the discharge, and a lesssharply pronounced break down avalanche, which favours paralleloperation of the auxiliary anodes. Experimentally it was found that 1-4auxilary anodes per cm. emitter surface are convenient numbers.

It has been found experimentally, that though small anodes of any of thetypes described show a positive characteristic when immersed into aplasma already formed, they nevertheless do not always operate inparallel in a stable way if the plasma is produced by their owndischarge. The reason is that the auxiliary anode which by some accidenthas struck first will produce a dense plasma only in its ownneighbourhood, but it produces a voltage drop in the supply, such thatthe reduced voltage is insufiicient for firing a second anode in aregion where there is no dense plasma. This means that only a fractionof the cathode area will be well supplied with ions. This unstablebehaviour is less pronounced if the auxiliary anode areas are very smallor if they are sharply pointed. It can be entirely suppressed byconnecting each auxiliary anode through a suitable resistor 5 to acommon terminal 6, as shown in FIG. 2. If for instance the current ofone auxiliary anode is 10 ma., a series resistance of about 200 ohms foreach is sufficient to stabilise the phenomenon The resistors can bedispensed with, however, if the electrodes 3 expose a sufiiciently smallarea to the discharge space. As an example, stable operation in parallelhas been achieved with electrodes as shown in the left position in FIG.3 with wires of 0.1 mm. diameter, exposing an area of 0.0078 mm. to thedischarge space at currents exceeding about 25 milliamperes perelectrode, without any series resistance.

It is found, however, that even a fully stabilised plurality ofauxiliary anodes gives only moderately good ratios of collector currentto auxiliary current if the last mentioned are operated from a directcurrent source. The reason is that by a general principle governing alldirect current discharges the voltage of the auxiliary anode relative tothe emitter adjusts itself to a minimum value, at which the ionisationis just sufficient to maintain the discharge with the given current. Inother words, the auxiliary current is maximised, while the collectorcurrent gets only a minimum of stray ions. This adverse be haviour canbe overcome by transient operation; by applying a large voltage andcurrent surge to the auxiliary anode and then breaking it off.

This operation is illustrated in FIG. 3. The auxiliary anodes areoperated by pulse circuits known by themselves with short, sharppositive pulses, occupying only a small fraction of the cycle, whileduring the major part of the cycle the voltage is negative, so that theauxiliary currents, of say, 10-20 times their mean value, flow onlyduring one-tenth or one-twentieth of the time. It has been foundexperimentally that the time T between two pulses can be anything belowabout 100 microseconds. With emitter-collector distance of the order 12mm. the main current after a pulse is observed to remain almost constantfor 50200 microseconds, depending on the gas pressure, after whichapproximately exponential decay starts with a time constant of the orderof 50200 microseconds. If therefore the pulses with a frequency of theorder of 10 kilocycles are themselves modulated with technicalfrequencies of 5060 cycles, or even high frequencies up to about 1-2kilocycles, the device according to the invention may be used togenerate pulsed currents, which, by means well known in themselves, canbe put together to produce alternating currents of rectangular orsinusoidal shape.

FIG. 4 is a diagram illustrating the collector currentcollector voltagecharacteristics of a device according to the invention, at some constantmean auxiliary current and at zero auxiliary current. The device acts asa generator when the collector-emitter voltage V is negative. It iscontrollable up to a voltage at which an arc discharge sets in betweenthe emitter and the collector. This is indicated in FIG. 4 by thebackward curving branch of the characteristics. When the auxiliaryanodes are not excited the arc will strike at about 12-20 volts inargon, at 8-12 volts in xenon, which gives a suflicient margin foroperation with alternating current, in which the collector current mustbe suppressed during one phase of the cycle.

The collector current-collector voltage characteristics have similarshape whether the auxiliary anodes are energised with direct current orwith pulses, but the auxiliary currents required for drawing a certaincollector current may be ten times or more larger in DC. operation thanin pulse operation, while the auxiliary voltages required are less thandoubled.

The realisations shown in FIGS. 1 and 2 have the disadvantage that whenapplied to directly cooled collectors, they require a high number ofreliable metal-ceramic or metal-enamel vacuum-tight seals. Therealisations shown in FIGS. 5-8 are free from this disadvantage, as thesystems of auxiliary anodes are completely inside the vacuum space, andrequire only one terminal.

FIGS. 5a-5g show the stages of manufacture of one such system. In FIG.5a, 7 is a strip of a metal, which as shown in FIG. 5a, is bent into theshape of an angle, with a rounded corner. This is then coated except inareas 8, shown in FIG. 5a with an insulating layer. One suitable methodof insulation is fire-enamelling, another is spraying with a ceramicsuch as alumina by means of a plasma gun. If these methods are used, theareas 8 are scraped bare of the insulation. In the next operation, shownin FIG, 5d, the central part of the angle is coated with a resistivelayer 9, preferably by vacuum evaporation of a nickel-chromium alloy. Inthe operation a metal tape 10 is prepared on which short lengths ofrefractory metal wires, e.g. of tungsten, molybdenum or tantalum arewelded at right angles, and the ends of these are introduced into theangle, which is then clamped tight around the wires, preferably betweenjaws coated with an elastic material, such as rubber. The result isshown in FIG. 5 In the next operation the tape is cut off, and theprojecting wires are cut to the required length. In order to removescratches which may have arisen during the clamping, the whole productwith the exception of the wires may be re-insulated, e.g. by spraying.The projecting wires can be protected during this operation by a resist.

In a system as described in FIG. 5 the current to the auxiliary anodesis carried by the U-shaped metal trough, which makes contact with theresistor layer only at the patches 8. The wires in turn make contactwith this strip midway between two patches 8, so that they are suppliedwith current from both sides, and the resistance in series with eachwire is one-quarter of the resistance between two patches 8. Theresistance required for stabilisation depends on the area which isexposed to the discharge. If this area is made very small, of the orderof one-hundredth of a square millimetre, stable parallel operationbecomes possible without stabilising series resistances. In this casethe design can be simplified. Instead of coating the inside of theangle-strip with a resistive material, a zone can be left bare, so thatthe wires make direct contact with the strip. Alternatively, the U-shaped strip is completely coated with an insulator, and the tape 10 isinserted into it, with the wires pointing outwards, and it is this tape10 which serves as the common conductor for the auxiliary anodes. Foradded safety, it is preferable to coat this tape too with an insulator,leaving only the projecting part of the wires bare.

FIGS. 6a-6c show another variety of an auxiliary anode system, in whichthe auxiliary anodes are not wires but metallic patches behind aperforated insulator. In FIG. 6a the metal strip is perforated with fineholes 12. This is then treated as before (FIG. 6b), but with theexception that the resistive layer is deposited on one side of the angleonly. Also it is preferable to apply a deposit of refractory metal orsmall metal plates 13 opposite to the holes, lest the resistive layermight be destroyed by the discharge through the holes. FIG. 60 shows thefinished product. The whole surface is insulated except the metalpatches which are exposed through the perforations.

FIGS. 7 and 8 show the application of these auxiliary anode systems tothe collectors 2. In FIG. 7 the auxiliary anode strips are inserted intoslots, and their ends are connected to a common lead to the terminal.All leads must be carefully coated with an insulator, because any barepatch could attract the discharge away from the small spikes. This canbe done by coating with a ceramic cement paste, but also by making allleads of tantalum and anodising the assembled system until every patchof bare metal is covered up by an insulating layer.

The perforated strips whose making was explained in FIG. 6 can be laidflat on the collector, and tied down for instance by metal strips weldedacross them in a sufficient number of places. In the example shown inFIG. 8 the whole strip is wound in a helix around the tubular collector2, which carries the coolant.

We claim:

1. A thermionic generator comprising an electron emitter and a collectorelectrode spaced apart from each other to define a discharge spacebetween them, a plurality of auxiliary anodes provided in said dischargespace, and pulse generator means for applying to said auxiliary anodespositive pulses separated by negative potentials of durationinsufiicient to allow appreciable decay between said positive pulses ofcurrent between said emitter and collector electrodes.

2. The generator as claimed in claim 1 in which the pulse generatormeans is adapted to amplitude modulate the positive pulses to producecorresponding modulation of the main discharge current.

3. A thermionic generator comprising an electron emitter and collectorelectrode spaced apart from each other to define a discharge spacebetween them, a plurality of metallic strips folded double along theirlengths and coated at least over their external surfaces with aninsulating layer and secured to the collector electrode, and a pluralityof short wires held between the two halves of the strips, which wiresproject into said discharge space and form auxiliary anodes.

4. A thermionic generator comprising an electron emitter and collectorelectrode spaced apart from each other to define a discharge spacebetween them, a plurality of metallic strips folded double along thedirection of their lengths and coated over both their internal andexternal surfaces with layers of insulating material except ReferencesCited UNITED STATES PATENTS 2,239,694 4/1941 Bennett 313-351 X 2,607,0168/ 1952 Kennebeck 313351 X 2,959,704 11/1960 Snell et al 313-351 X3,021,472 2/ 1962 Hernquist 313230 3,112,863 12/1963 Brubaker et a1313217 3,238,395 3/1966 Sense 313310 2,697,800 12/1954 Roberts 313351 XFOREIGN PATENTS 29,854 7/ 1959 Germany.

JOHN W. HUCKERT, Primary Examiner. A. I. JAMES, Assistant Examiner.

